Abstract:

Solid phosphates Li3PO4,BiPO4 and Li3PO4•BiPO4 prepared by precipitation were used as the surface materials. Their surface composition, structure and reaction behavior have been investigated by the techniques of XRD,IR,TPD and laser stimulated surface reaction (LSSR).In solid phosphates, there are surfaces Lewis base sites P=O and Lewis acid sites Mn+. Two types of methanol adsorption species have been found, the molecular type consists of the adsorption of the H of C-H bond on the Lewis base site P=O and the dissociative type that of O and H of O-H bond on the Lewis acid site Mn+ and Lewis base site P=O separately. Because of the interaction of Li+ and Bi3+,the introduction of Li3PO4 into BiPO4 can promote the molecular type adsorption and reduce the dissociative type adsorption. On the selective excitation of the surface P=O bond by 1077cm-1 laser photons, the activated terminal oxygen reacts with the adsorbed H of C-H bond forming CH2OH and P-OH. The former species then transfers to the Lewis acid site Mn+ and ethanediol is produced by the coupling reaction. The surface hydroxyl P-OH reacts with each other to form H2O and the surface oxygen defect. The Lewis acid sites Mn+ on the surface of solid materials can chemisorb gaseous molecular oxygen continuously and transform it into O2-to compensate the consumed surface lattice oxygen. Under atmospheric pressure and at the tempurature of 120℃,the methanol conversion of 16% and ethanediol selectivity of 97.7% can be obtained by using 1077 cm-1 laser exciting the surface P=O bond on Li3PO4•BiPO4 for 1000 illuminating times. Increase of temperature has the effects of compensating the energy density of laser photon and enhancing the desorption of reaction products. But it also causes the formation of dimethyl ether by thermal catalytic reaction while the temperature is over 120℃.

Key words: Laser stimulated surface reaction, Oxidative coupling, Methanol, Ethanediol, Complex phosphates